The present invention relates to an arrangement for suppressing unwanted electromagnetic wave radiation from unavoidable openings in shielded enclosures, and more particularly to depressing such radiation in data recorders which generate a wide spectrum of radiation frequencies in response to the processing of digital information such as coded pulse groups.
In data processors, various kinds of signals are involved. For example, electrical currents are generated which are associated with the operation of the power supply, with the operation of solenoids and with the occurrence or processing of coded pulse groups representing data. As is well known, recurrent pulses of the type handled in data processing equipment have associated therewith a harmonic frequency content sufficient to produce electromagnetic radiation over a wide spectrum of frequencies. Under certain circumstances it is desirable that such radiation be limited to the shielded enclosure surrounding the data processor and not be permitted to escape through unavoidable openings in the processor for interference with the operation of other equipment.
Attempts to control this radiation have been relatively unsuccessful in the past. For example, attempts to seal the opening by flaps, etc., have proved unsuccessful because the flap has to accommodate the width of the record medium which in most cases is a continuous sheet of paper. It is this paper width at the opening that controls the amount of electromagnetic radiation that is permitted to escape from the shielded enclosure and not the height of the opening. Attempts to merely coat the inside surface of the opening with a lossy material have been relatively unsuccessful because of the substantial thickness of coating that would be required. Other schemes based on controlling the opening size to extremely close tolerances and dimensions have resulted in interference with proper paper movement. Also, efforts to insert dielectric materials into the waveguide to absorb radiation have proved costly and difficult to implement because of the tendency for this approach to produce undesirable standing waves which is contrary to the objective of suppressing as great a spectrum of frequencies as possible.